Microelectromechanical structures (MEMS) and other microengineered devices are presently being developed for a wide variety of applications in view of the size, cost and reliability advantages provided by these devices. Many different varieties of MEMS devices have been created, including microgears, micromotors, and other micromachined devices that are capable of motion or applying force. These MEMS devices can be employed in a variety of applications including but not limited to hydraulic applications in which MEMS pumps or valves are utilized and optical applications which include MEMS light valves and shutters.
MEMS devices have relied upon various techniques to provide the force necessary to cause the desired motion within these microstructures. For example, cantilevers have been employed to apply mechanical force in order to rotate micromachined springs and gears. In addition, some micromotors are driven by electromagnetic fields, while other micromachined structures are activated by piezoelectric or electrostatic forces. Recently, MEMS devices that are actuated by the controlled thermal expansion of an actuator or other MEMS component have been developed. For example, U.S. patent application Ser. Nos. 08/767,192; 08/936,598, and 08/965,277 which are assigned to MCNC, the assignee of the present invention, describe various types of thermally actuated MEMS devices. The contents of each of these applications are hereby incorporated by reference herein. Thermal actuators as described in these applications comprise arched beams formed from silicon or metallic materials that further arch or otherwise deflect when heated, thereby creating motive force. These applications also describe various types of direct and indirect heating mechanisms for heating the beams to cause further arching, such that the thermal actuator structures move relative to other microelectronic structures when thermally actuated.
In practically every application of MEMS devices, precisely controlled and reliable movement is required. Given the micron scale dimensions associated with MEMS structures, stable and predictable movement characteristics are critically important. The movement characteristics of MEMS devices can be affected by intrinsic factors, such as the type of materials used to fabricate the MEMS device, the dimensions and structure of the MEMS device, and the effects of semiconductor process variations. All of these intrinsic factors can be controlled to some extent by the MEMS design engineer. In addition, movement characteristics may be affected by extrinsic factors such as fluctuations in the ambient temperature in which the MEMS device operates, which cannot be controlled by the MEMS design engineer. While all of the above factors affect the ability of a MEMS device to move precisely and predictably, the impact of these factors may vary from device to device. For instance, while thermally actuated MEMS devices are affected by all the above factors, they are particularly sensitive to ambient operating temperature variations because they are thermally driven devices.
More particularly, a thermally actuated MEMS device may operate unpredictably or erroneously since the MEMS device will move not only in response to thermal actuation caused by active heating or cooling, but also due to changes in the ambient operating temperature. If ambient temperatures are very high, parts of a MEMS device designed to move in response to thermal actuation may move too much or too far. Alternatively, in very low ambient temperatures, parts of a thermally actuated MEMS device designed to move may not move sufficiently in response to thermal actuation thereof. In either temperature extreme, maintaining parts of MEMS device in predictable positions relative to each other can be difficult. Ambient temperature effects can thus affect the reliability and limit the possible applications of MEMS thermally actuated devices. Those skilled in the art will appreciate that similar problems can arise due to residual stress created by semiconductor process variations and structural differences within MEMS devices.
Therefore, while some thermally activated MEMS structures have been developed, it would still be advantageous to develop other types of thermally actuated structures that would operate more reliably or more precisely even when exposed to significant ambient temperature fluctuations. Consequently, these MEMS structures would be suitable for a wider variety of applications. Numerous applications including but not limited to switches, relays, variable capacitors, variable resistors, valves, pumps, optical mirror arrays, and electromagnetic attenuators would be better served by MEMS structures with these attributes.